The present invention relates to a semiconductor integrated circuit and an operation method of the same and, more particularly, to a technique effective to reduce the possibility that a DC-DC converter stops in the case where supply power is small and load current such as charging current of a secondary cell (battery) is large.
Conventionally, a semiconductor integrated circuit and an antenna coil are mounted on an IC card, and power supply of the IC card is performed by receiving an RF signal from a reading/writing device called a card reader/card writer by the antenna coil and rectifying it by a rectifier circuit. An IC card having no power supply as described above is spread in an automatic ticket gate system, electronic money, logistics, and the like. The RF power supply is performed to the IC card, and unique identification information (ID information) is stored in a built-in nonvolatile memory, so that the IC card is called an RFID card. An IC card used in the fields of an automatic ticket gate system, electronic money, and the like uses NFC (Near Field Communication) using an RF frequency of 13.56 MHz.
On the other hand, a wireless power supply system called “wireless charger” capable of charging a portable device such as a smart phone which is just put on a dedicated charging table without coupling a power cable to the portable device is being spread. The wireless power supply system is provided to address draining of the battery of a cellular phone called a smartphone. That is, a smartphone is a multifunctional cellular phone having an affinity for the Internet and whose base is the function of a personal computer or a multifunctional cellular phone obtained by adding the PDA function to a telephone and mail function. A wireless power supply system is based on the international standard called “Qi” developed by Wireless Power Consortium (WPC) as an industry organization. By providing each of a transmission-side device and a reception-side device with a coil, power can be supplied from the transmission-side device to the reception-side device by the electromagnetic induction method. The advantages of the wireless power supply system are that it is unnecessary to insert and remove a power connector for charging and, particularly, a work of opening/closing a cover of a power supply connector of a portable device can be omitted.
On the other hand, it is written in FIG. 2 of the following patent literature 1 and disclosure related to it that NFC is performed between a port device and a mobile device and non-contact power transmission for charging a secondary cell (battery) of the mobile device is performed from the port device. The mobile device has an induction coil for NFC and an induction coil for charging, the induction coil for NFC is coupled to an NFC chip, and the induction coil for charging is coupled to a power receiver for charging, a charge controller, and a secondary cell. The port device has an induction coil for NFC and an induction coil for charging, the induction coil for NFC is coupled to an NFC chip, and the induction coil for charging is coupled to a power supply unit for charging.
It is written in FIG. 3 of the patent literature 1 and disclosure related to it that an operation timing of the NFC between a port device and a mobile device and an operation timing of non-contact power transmission for charging a secondary cell (battery) of the mobile device from the port device are repeated in a time division manner. Since the non-contact power transmission for charging is not performed at the operation timing of the NFC by the time division, it is presumed that deterioration in the signal quality such as noise from the non-contact power transmission to the NFC can be lessened.
Further, another non-contact power transmission method is written in FIG. 7 of the patent literature 1 and disclosure related to it that NFC is performed between a port device and a mobile device and a secondary cell (battery) of the mobile device is charged from the port device. The mobile device has one induction coil used for both NFC and charging, the one induction coil is coupled to a circuit selector, and the circuit selector is coupled to an NFC chip and a power receiver for charging. The circuit selector selects arbitrary one of the NFC chip and the power receiver for charging, and the selected one is coupled to the single induction coil via the circuit selector. The port device has one induction coil used for both NFC and charging, the one induction coil is coupled to a circuit selector, and the circuit selector is coupled to an NFC chip and a power supply unit for charging. The circuit selector selects arbitrary one of the NFC chip and the power supply unit for charging, and the selected one is coupled to the single induction coil via the circuit selector.
Further, the following patent literature 2 discloses that, in an electronic device for charging a battery by being selectively coupled to power supplies of two or more kinds, a controller is used which performs prompt coupling to another power supply to start charging of the battery when coupling to the power supply from which power is supplied is cancelled. Specifically, the control by the controller is performed in such a manner that the battery is charged by an AC power supply when current is supplied from the AC power supply to an AC coupling unit, and the battery is charged by a power supply of an external device when current is not supplied from the AC power supply to the AC coupling unit but current is supplied from an external device to an external device coupling unit. In particular, when the external device coupling unit is coupled to an external device during the battery is charged by the AC power supply, the controller performs initial communication with the external device and makes a charging setting necessary to charge the battery via the external device. The external device coupling unit is concretely a USB coupling unit, and an interface of another standard such as IEEE1394 can be also employed. In the case where the electronic device is coupled to both the AC power supply and the external device, current from the AC power supply is larger than that from the external device, so that the controller charges the battery by the AC power supply.
Further, the following patent literature 3 discloses that, to save power of a power supply of an electronic device and simplify control of a power supply circuit and the like, a control unit of a DC-DC converter which supplies operation power for a system device and a charging power for a battery controls so that the sum of the operation power for the system device and the charging power for the battery is almost constant. The system device includes a CPU, a hard disk drive, a CD-ROM drive, a display unit, and the like.